The surface quality of continuously cast steel slabs, which are subsequently rolled to steel sheet is one of the most important attributes that determine the sheet product quality in the modern steel industry. In the extremely competitive steel industry, the cast surfaces have to be essentially free of all defects, particularly in plants having near net shape and thin slab casting.
Surface defects are related to a number of factors, such as entrapped flux, solid and liquid inclusions, surface and subsurface cracking, and mold oscillation marks. Many studies have been made seeking the reasons and remedies to correct particularly such defects as longitudinal cracks and mold oscillation marks. Applicant has discovered that one of the factors causing these defects is the formation of a solidified rim of slag at the top portion of the casting mold, due to the heat loss to the mold and to the environment of the liquid slag formed by the melting mold powder.
Mold powders provide chemical and thermal insulation to the molten steel surface and control the heat flow and friction between the mold and the initial steel shell formed as the slab undergoes solidification. The rapid cooling exerted by the water-cooled mold on the liquid steel in order to promote solidification thereof, results in steep temperature gradients in the solid shell generating thermal strains as the shell expands and contracts. Also the semisolid steel is subjected to mechanically induced stresses by the friction of the steel with the mold (including vertical oscillations). Any of these stresses and strains at the initial solidification of the shell may result in crack formation on the slab surface.
Mold powder melts to form a layer of liquid slag between the solidifying steel and the mold, which controls friction and also insulates the liquid steel and molten powder slag at the top of the mold. Typical operating practice for addition of mold powder is described by Rama Bommaraju, "Optimum selection and application of Mold fluxes for carbon steels", presented at the 74th Steelmaking Conference of the Iron and Steel Society, in Washington, D.C., on Apr. 14-17, 1991 (incorporated herein by reference). In this article the author discourages the tendency of operators to add large quantities of mold powder and wait until it turns red-hot and then make the next addition. The author concludes that in most instances, the liquid layer of slag should be maintained above 6 to 12 mm. This can usually be achieved by maintaining a minimum of 25 mm thickness of unreacted solid powder layer. The author recognizes that the powder also thermally insulates the slag and prevents it from freezing in the mold which may cause freezing of the steel meniscus and other problems. The author suggests to cascade the powder over the edge of the mold and to maintain the area between the mold wall and the shroud of the Submerged Entry Nozzle (SEN) always covered with a 25 to 50 mm (1-2 inches) thick powdered layer, plus perhaps another 1-2 inches during start-up or tundish or tube changes.
It has been found that the current practices, including the aforementioned mold powder addition, is not satisfactory to increasingly demanding customers and that such practices are still causing many defects in the products, as for example longitudinal cracks and oscillation marks. The technical literature in this field is full of discussions and explanation of the phenomena involved in the early solidification stages of continuously cast slabs, as well as including many proposals and countermeasures to minimize such quality problems. For example increase the mold oscillation frequency, to increase the downward speed of the mold, to insert an area of chromium carbide at the top area of the mold in order to create a "hot spot" thus decreasing the extension of such solidified rim, etc.
Another proposal to improve the surface quality is to apply electromagnetic or magneto-hydrodynamic forces to generate stirring of the liquid steel and consequently to improve the uniformity of heat transfer and solidification.
None of the above techniques however has been completely successful in the elimination of longitudinal cracks and oscillation marks, and the technical literature accepts the problem of slag rim formation as an unavoidable consequence of the mold cooling. With the method of the present invention it has been found that the solidified slag rim is eliminated and the no longitudinal cracks have been detected in the product.